U.S. Pat. No. 3,988,774 describes the coating of an intermediate layer of HgCdTe onto a photoconducting layer of HgCdTe. In one special case (col. 2, lines 31-33), the patent indicates that the concentration of mercury in the intermediate layer could be zero; in this case, the intermediate layer is CdTe as in the present invention. However, the patent teaches away from the present invention because the methods of applying the intermediate layer mentioned in the patent are cathodic sputtering, epitaxy, and vacuum evaporation (col. 2, lines 47-49), not electrochemical deposition as in the present invention. Cathodic sputtering is unacceptable for coating with CdTe because it would damage the HgCdTe surface. Furthermore, it is not an in situ process as that of the present invention, and therefore, impurities can more readily be introduced. Vapor phase epitaxy and liquid phase epitaxy are unacceptable for coating HgCdTe with CdTe because they are high temperature processes, and therefore would cause interdiffusion between the CdTe and the p-type HgCdTe, undesirably allowing mercury to deplete out of the HgCdTe. The growth of CdTe on HgCdTe using molecular beam epitaxy (MBE) has been reported in Migliorato et al., "CdTe/HgCdTe Indium-Diffused Photodiodes", Infrared Phys., Vol. 22, pp. 331-336 (1982). The methods of electrochemical deposition described in the instant patent application offer the following advantages over MBE: lower cost, superior control over resistivity and conductivity type, and ability to clean the surface of the HgCdTe in a non-destructive manner. Conventional vacuum evaporation is not a viable process because of contamination problems.
Chu et al., "Liquid phase epitaxial growth of CdTe/Hg.sub.1-x Cd.sub.x Te multilayers (0.3&lt;x&lt;0.5)," J.Appl.Phys. 51 (4), April 1980, pp. 2255-2257, describes the coating of CdTe on HgCdTe using liquid phase epitaxy. As stated previously, this high temperature process causes an unacceptable level of interdiffusion.
Panicker et al., "Cathodic Deposition of CdTe From Aqueous Electrolytes", J.Electrochem.Soc. Vol. 125, No. 4, pp. 566-572, April, 1978; and Rod et al., "Emerging Materials for Solar Cell Applications--Electrodeposited CdTe", Final Report, Feb. 14, 1979--Feb. 14, 1980, Contract No. AC04-79ET23008, U.S. Dept. of Energy, pp. 5-9, describe the cathodic deposition of CdTe on nickel or glass cathodes for photovoltaic cells, but not the deposition of CdTe on HgCdTe as in the present invention.
Basol et al., "Observation of Electron Traps in Electrochemically Deposited CdTe Films", Solid State Electronics Vol. 24, pp. 121-125 (1981); and Fulp et al., "High-efficiency electrodeposited cadmium telluride solar cells", App.Phys.Lett. 40(4), Feb. 15, 1982, pp. 327-328, describe the manufacture of photovoltaic cells using the method described in Panicker et al., supra.
Several references discuss the passivation of HgCdTe by anodic oxidation, e.g., U.S. Pat. No. 3,977,018. Although an electrochemical process, in anodic oxidation the passivation layer is a HgCdTe oxide of varying composition, not CdTe as in the present invention; and the photoconductive HgCdTe is the anode, not the cathode as in the present invention.
Several references discuss the passivation of HgCdTe using zinc sulfide, e.g., Davis et al., "Properties of passivant films on HgCdTe--interaction with the substrate", Proceedings of SPIE--The International Society for Optical Engineering, Vol. 285, "Infrared Detector Materials", Apr. 20-24, 1981, pp. 126-134. This process is more difficult to control reliably than the passivation with CdTe described in the present specification, and does not provide as good lattice matching.
Rhiger et al., "Exploratory Development on PV HgCdTe Surface Leakage", Interim Report for Period July 1, 1980 to June 30, 1981, Contract No. F33615-80-C-5084, prepared for Materials Laboratory at Wright-Patterson AFB, Nov. 8, 1981, describes on page 60 the passivation of HgCdTe using native oxides and silicon oxide. However, the lattice structure is not matched as in the present invention.